1. Technical Field
The present invention relates to an oscillating device, an oscillating element and an electronic apparatus.
2. Related Art
In a TCXO (Temperature Compensated X′tal Oscillator), high frequency stability is obtained by cancelling a shift (frequency deviation) of the oscillation frequency of a quartz crystal resonator from a desired frequency (nominal frequency) in a specified temperature range. For example, the frequency stability of the general TCXO is about ±200 ppb (use temperature range: −20° C. to +70° C.).
As shown in FIG. 12A, an AT-cut quartz crystal resonator in which the frequency temperature characteristic is approximated by a cubic function is generally used in the TCXO. However, the cubic functions of the respective AT-cut quartz crystal resonators are different from each other. Thus, in characteristic inspection of the TCXO, a relation between temperatures at four or more points and oscillation frequencies is obtained, and information (temperature compensation information) for temperature compensation of the frequency temperature characteristic of the quartz crystal resonator is required to be calculated.
When the TCXO operates, a temperature compensation voltage to generate a frequency change as shown in FIG. 12B is internally generated based on the temperature compensation information, and the frequency temperature characteristic of an oscillation signal to be outputted is made to approach a flat characteristic.
In recent years, in a small base station for a small call area such as a femto cell or a measurement device, a more highly accurate clock signal is required. In such use, a frequency stability of, for example, about ±50 ppb is often required.
As an oscillating device with excellent frequency stability as compared with the TCXO, there is an OCXO (Oven Controlled Crystal Oscillator) provided with a constant temperature oven. However, since the constant temperature oven and a temperature control circuit thereof are provided, there is a defect that the OCXO is expensive and is large as compared with the TCXO.
According to the invention disclosed in JP-A-2010-124348 (patent literature 1), a base mounted with a TCXO is sealed with a cover and the temperature is kept constant. This has a middle temperature characteristic between the TCXO and the OCXO, and an inexpensive and small oscillating device is provided.
However, also in the method of patent literature 1, a temperature compensation voltage as shown in FIG. 12B is internally generated with respect to temperature change, and the frequency temperature characteristic of an oscillation signal to be outputted is made flat. Thus, at the time of manufacture, such an operation is required that high-order functions different among the respective AT-cut quartz crystal resonators are obtained, and for example, an adjustment is made so that a local maximum value and a local minimum value are offset by each other. At this time, it is necessary to obtain a relation between temperatures at four or more points and oscillation frequencies, and the efficiency of manufacture is reduced.
Besides, in the method of patent literature 1, a closed space formed by sealing the base with the cover functions as a pseudo oven (hereinafter, the term “oven” includes the pseudo oven of the closed space as stated above). At this time, it is necessary to keep the oven at a specified temperature with high accuracy (for example, ±1° C.) comparable to the OCXO. However, in the oscillating device of patent literature 1 which is constructed by assembling the TCXO and other circuits, a variation between individuals is liable to occur. Thus, the adjustment operation of the temperature control circuit is required in order to adjust the temperature of the oven to a target temperature. However, in order to keep the temperature of the oven in a narrow range of, for example, ±1° C., the adjustment operation becomes very troublesome.
As described above, in the method of patent literature 1, the adjustment operation before shipment takes time, the efficiency of manufacture is reduced, and consequently, there is a high possibility that the cost rises.